532 Bull. Korean Chem, Soc. 1996, VoL 17, No. 6 Hee-Gweon Woo et al.
Photopolymerization of Methacrylic Acid with Secondary Silanes
Hee-Gweon Woo*, Sun-Hee Park, Lan-Voung Hong, Soo-Yeon Yang, Haeng-Goo Kang, and Heui-Suk Ham
Department of Chemistry, Chonnam National University, Kwangju 500-757, Korea Received March 16, 1996
The bulk photopolymerization of methacrylic acid (MA) with secondary silanes such as PhMeSiH? and PhzSiH? gave poly(MA)s possessing the secondary silyl moiety presumably as an end group. It was found that while the polymeriza
tion yields and intrinsic viscosities decreased, the TGA residue yields and the relative intensities of SiH IR stretching bands increased with increasing mole ratio of the secondary silane over MA. The sterically less bulky silane PhMeSiH?
produced poly(MA)s with somewhat higher molecular weights and with similar TGA residue yields., compared with the sterically bulkier silane Ph2SiH2. The secondary silanes seem to significantly influence on the photopolymerization of MA as both chain initiation and chain transfer agents.
Introduction
Photopolymerization technology applicable conveniently is commercially and amply used today in the areas of surface coatings, photoresists, adhesives, and holography. A wide var
iety of unsaturated vinyl derivatives can be polymerized via a free-radical chain process.1 Albeit any vinyl derivative that will undergo chain polymerization is, in principle, subject to photopolymerization or photosensitized polymerization, only a few unsaturated compounds are known to absorb 250- 500 nm wavelength light which is the most convenient wave
length range for experimental work. The detailed mechanism forming the propagating radicals is not completely unders
tood, but it appears to involve the conversion of an electroni
cally excited singlet state of the monomer to a long-lived excited triplet state.2 The capability performing a thermody
namically possible polymerization counts upon its kinetic fea
sibility on whether the process proceeds at a reasonable rate under a provided set of reaction conditions. Hence, an initia
tor (or promotor) is often needed to attain the kinetic feasi
bility.
Hydrosilanes have taken part in versatile catalytic reac
tions such as free radical reduction of organic halides? nu
cleophilic reduction of carbonyl compounds,4 dehydropolyme
rization,5 cross-dehydrocoupling,6 and hydrosilation of olefins.7 The hydrosilation has been applied to prepare many interes
ting types of silicon containing polymers such as dendrimers8 and copolymers.9 We reported the bulk phofopolymeriz저ti°n of methyl methacrylate (MMA) with various silanes to pro
duce poly(methyl methacrylates), poly(MMA)s containing the corresponding silyl moiety probably as an end group.10 We recently described the bulk photopolymerization of methacr
ylic acid (MA) with primary silane.11 In the present paper we wish to report the bulk photopolymerization of MA with secondary silanes giving poly(methacrylic acids), p이y(MA)s containing the silyl moiety presumably as an end group in order to compare the silane effect on the photopolymeriza
tion of MA.
Experimental Section
Materials and Instrumentation. All reactions and
manipulations were performed under prepurified nitrogen using Schlenk techniques. Dry, oxygen-free solvents were used throughout. Glassware was flame-dried or oven-dried before use. Infrared spectra were obtained using a Nicolet 520P FT-IR spectrometer. Proton NMR spectra were recor
ded on a Bruker ASX 32 (300 MHz) spectrometer 낞sing DMSO-d6/DMSO-H6 as a reference at 2.49 ppm downfield from TMS. Reduced viscosity (
门
湖) and inherent vi용cosity(叫心) of different concentration (c in g/dL) of p이ymer solu
tions in DMF were obtained by measuring three satisfactory readings of the efflux time (p이ymer, t; solvent, to) with an Ostwald-Fenske viscometer immersed in the constant-tempe
rature bath maintaining at 25± 0.01 t and by substituting the mean of three readings into the known equations.12 The extrapolation of the two viscosities to the same intercept as c approached to zero afforded the intrinsic viscosity [r口 in dL/g. Thermogravimetric analysis (TGA) of p이ymer sam
ple was performed on a Perkin Elmer 7 Series thermal anal
ysis system under a nitrogen flow (50 mL/min). The polymer sample was heated from 25 to 700 t at a rate of 10 C/min.
TGA residue yi인d (for convenience sake, read at 400 t) is reported as the percentage of the sample remaining 거fter completion of the heating cycle. For the photolysis experime
nts a Raynot photochemical reactor (mod이 RPR-2080) m거de by the Southern N. E. Ultraviolet Co., which has merry-go- round system in order to uniformly irradiate all samples, was used. The built-in monochromatic UV light sources (RUL-300 nm UV lamp; lamp
订
itensity=693X 1(严
hv mL"1 min-1) was positioned approximately 17 cm from the reaction quartz tube. Methacrylic acid (Aldrich Chemical Co.) was saturated with NaCl (to remove the bulk of the water), then나le organic phase was dried with CaCh and distilled under vacuum before use. PhMeSiH2 and Ph2SiH2 were prepared by reduction of PhMeSiCL and PhzSiCb (Aldrich Chemical Co.), respectively, with LiAlR (Aldrich Chemical Co.) in die
thyl ether.13
Bulk Photopolymerization of MA with Phenylmeth- ylsilane. Bulk photopolymerization of MA with different mole ratio of PhMeSiH2 (10 :1 through 3 : 7) was performed.
In a typical experiment, a quartz test tube (1 cmX20 cm) charged with MA (1.72 g, 20 mmol) and PhMeSiH2 (0.24 g, 2.0 mmol) was degassed, sealed, and irradiated with 300
Bull. Korean Chetn. Soc. 1996, Vol. 17, No. 6 533 Ph
사
oMlymeristion of MA with Secondary Silanesnm UV-light for 6 h. The polymer was taken in methanol, precipitated in diethyl ether, filtered off, and dried to give 1.67 g (85%) of white solid (TGA residue yield at 400 °C:
62%).NMR (& DMSOd, 300 MHz): 0.35 (m, Si-CH3), 0.9-1.1 (br, 3H, C-CH3), 1.8-2.1 (br, 2H, CH2), 72-7.7 (m, ArH), 12.3 (br, 1H, OH). The SiH resonance was not clearly obser
ved in the NMR spectrum for uncertain reasons. IR (KBr pellet, cm-1): 3400 br s (v0-h), 2168 w (vSi-H). 1715 s $(vc=o)・
Intrinsic viscosity: 0.62 dL/g.
Bulk Photopolymerization of MA with Diphenylsi
lane. Bulk photopolymerization of MA with diff은rent mole ratio of Ph2SiH2 (10 : 1 through 3 : 7) was carried out. The following procedure is representative of the polymerization reactions. A quartz test tube (1 cmX20 cm) was loaded with MA (1.72 g, 20 mmol) and Ph2SiH2 (0.37 g, 2.0 mmol). The mixture was degassed, sealed, and UV-irradiated for 6 h.
The polymer was dissolved in methanol, precipitated in die-
나lyl ether, filtered off, and dried to give 1.94 g (93%) of white solid (TGA residue yield at 400 °C: 60%).NMR (8, DMSO-d6t 300 MHz): 0.9-1.1 (br, 3H, C-CH3), 1.8-2.1 (br, 2H, CH2), 7.3-7.7 (m, ArH), 12.3 (br, 1H, OH). The SiH reso
nance was not clearly observed in the〔H NMR spectrum.
IR (KBr pellet, cm'1): 3400 br s (vo.H), 2140 w (vSi-H), 1704 s (vc=o)- Intrinsic viscosity: 0.38 dL/g.
Results and Discussion
The poly(MA)s containing phenylmethylsilyl moiety with intrinsic viscosities of 0.17-0.62 dL/g and TGA residue yields of 62-69% were prepared in 14-85% yields by bulk photopol
ymerization of MA with different mole ratio of phenylmethy
lsilane (MA : phenylmethylsilane = 10 :1 through 3 : 7). The polymers were soluble in DMF, DMSO, and methanol. The characterization data of the resulting poly(MA)s are summa
rized in Table 1.
The poly(MA)s containing diphenylsilyl moiety with intrin
sic viscosities of 0.19-0.38 dL/g and TGA residue yi이ds of 60-68% were also synthesized in 8-93% yields by bulk photo
polymerization of MA with different mole ratio of diphenylsi
lane (MA : diphenylsilane =10 : 1 through 3 : 7) (eq. 1).
Me
COOH (1)
The polymer응 were soluble in DMF, DMSO, and methanol.
The characterization data of the resulting poly(MA)s are lis
ted in Table 2.
It is known that high-molecular-weight polymer is formed instantly and that the weight average molecular weight gene
rally increases with increase of polymerization yield in the radical polymerization of vinyl monomers.1 The intrinsic vis
cosity is directly related to the weight average molecular weight of polymer.12 As shown in Table 1 and Table 2, while the polymerization yields and intrinsic viscosities ［
们
decreased, the relative intensities of SiH IR stretching bands and TGA residue yields increased as the mole ratio of silane over MA augmented. These facts can be rationalized as fol
lows (vide infra). The absorption of light may produce an
Table 1. Characterization of Photopolymerization of MA with Phenylmethylsilane*7
Mol ratio (MA:Silane)
Yield (%)
Intrinsic viscosity”
［们
R 시 ative intensity*
IR (VS1H)
TGA residue yield (%, at 400 °C)
10:1 85 0.62 1.0 62
7:3 43 0.36 1.9 67
5:5 31 0.27 2.7 68 -
3:7 14 0.17 3.2 69
UV-irradiation for 6 h. Measured in DMF at 25 unit, dL/g.
€ Relative ratio with respect to the intensity of 기siH (MA : phenyl- methylsilane —10 :1).
Table 2. Characterization of Photopolymerization of MA with Diphenylsilane"
Mol ratio (MA:Silane)
Yield (%)
Intrinsic viscosity7*
如
R 이 ative intensity IR (vSiH)
TGA residue yield (%, at 400 t)
10:1 93 0.38 1.0 60
7:3 47 0.24 1.8 66
5:5 31 0.21 2.7 67
3:7 8 0.19 3.2 68
UV-irradiation for 6 h. b Measured in DMF at 25 unit, dL/g.
f Relative ratio with respect to the intensity of vsiH (MA : diphen
ylsilane =10 :1).
excited singlet state of MA which will either fluoresce or be converted to an excited and long-lived triplet excited state, diradical of MA monomer. Attack on the other MA by this diradical affords a new diradical of MA dimer which either reverts to the ground state two MA m이ecules or atta
cks on the other MA that ultimately initiate polymerization.2 At neat condition the latter will be a predominant process to produce poly(MA) radicals. At high MA or low silane con
centrations, chain propagation will be able to compete with chain transfer over the poly(MA) radicals. However, the chain transfer will eventually rule over the chain propagation with increasing silane concentration. The chain transfer mi
ght produce a silyl radical which, in turn, leads to chain initiation, resulting in the production of poly(MA) containing the silyl moiety as an end group as shown in Scheme 1.
It is known that polysilyl radicals generated from the pho
tochemical homolysis of polysilanes may initiate the free-ra
dical chain polymerization of some vinyl monomers.14 The secondary silanes seem to considerably affect on the photo
polymerization as both chain initiation and chain transfer agents by operating competitively and simultaneously. The direct chain transfer constants of the secondary silanes for radical polymerization of MA are not available. Nonetheless, it could serve as an excellent chain transfer agent because PhSiH3 has low Si-H bond energy of 88.2 kcal/mol15 which is comparable to S-H bond energy of mercaptans, known to date to be one of most powerful chain transfer agents, of 87 kcal/mol.16 In fact, it has been reported that chain tran
sfer constant for radical polymerization of MMA at 60 t
534 Bull. Korean Chem. Soc. 1996, Vol. 17, No. 6 He6-Gweon Woo et al.
---► MA + hv,
hv
MA ----a (MA)* ——
--- a - CH2 — &Me(C그O)OH
-CH, — CMe(C=O)OH
(MA|/|R2SiH2|
chain propagation
R2HSi
R2SiH2 hv
chain initiation and propagation
、
，
、，
、，、，、，、，、，
COOH
■SiHR2
★ u.
(R2SiH2)* •' Me
COOH
Figure 1. TGA thermogram of p이y(MA) obtained from the pho
topolymerization of MA with phenylmethylsilane (mole ratio, 10 : 1).
Scheme 1. Postulated Mechanism for Photoreaction of MA with Secondary Silane.
is 2.7 for thiophenol and 0.12 for triphenylsilane.17 The Si- H bond energies of silanes are known to be mostly uniform except the silanes with strongly electron-withdrawing and/
or silyl substituents in the a-position.3 It is recently reported that the substitution of methyl group on a silane decreases the hydrogen donation ability of the silane, but the substitu
tion of phenyl group increases it.18 The hydrogen donation ability of a silane appears to be not related always to the Si-H bond energy of a silane. This might suggest that the aryl group of an arylsilane first absorbs the energy and then transfers it into the silicon center, which leads to the photo
chemical homolysis of Si-H bond. The energy transmission could be at short range. However, we are not sure of this hypothesis yet. A study for verifying the matter is in prog
ress using fluorophotometer.
As 아lown in Table 1 and Table 2, the sterically less bulky silane PhMeSiH2 afforded poly(MA)s with somewhat higher molecular weights than 나蛇 sterically bulkier silane Ph2SiH2.
The MA photopolymerization yields were higher than the MMA photopolymerization yields with the secondary silanes.”"
The polymerizability of MA by the silyl radical seems to be naturally higher than that of MMA. The TGA residue yields of poly(MA)s with the secondary silanes were much higher than those of poly(MMA)s with the secondary silanes.10 The change of the TGA residue yields of the p이y(MA)s with increasing the relative silane concentration was very little when compared to that of the poly(MMA)s. It is interesting to note that while the weight loss of the p이y(MMA)s was smoothly occurred up to 350 °C with few turning points, the abrupt weight loss of a narrow range of the poly(MA)s was occurred twice at the turning points around 150 t and 250 t before 350 °C (Figure 1), although all the p어y(MA)s and poly(MMA)s rapidly and exhaustively decomposed after 350 t.
We believe that cross-linking reactions could be happened by 나le acidolysis between Si-H group and acid group in the different polymer chains at around 150 t, yielding a silyl
ester linkage19 and then by the dehydration between two acid groups in the different polymer chains at around 250 C forming an acid anhydride linkage.20 For the secondary silanes the little change of TGA residue yields at 400 of the poly(MA)s with increase of the relative silane concent
ration may suggest that the cross-linking by the formation of acid anhydride linkage should contribute by far more than the cross-linking by the formation of silyl ester linkage. The silyl ester linkage is known to be thermally weak. The cross
linking possibility might be slim un the photopolymerization conditions. The cross-linking processes could require high energy, which are anticipated only to occur during the pyrol
ysis.21 However, we 아lould admit at this moment that we cannot completely exclude the low degree of cross-linking possibility during the photopolymerization.
In conclusion, this work describes the photopolymerization of MA with secondary silanes such as phenylmethylsilane and diphenylsilane. While the p이ymerization yields and int
rinsic viscosities of the poly(MA)s containing the secondary silyl moieties decreased, 나le TGA residue yields and intensi
ties of SiH stretching IR bands increased as the m이e ratio of the secondary silanes over MA increased. The p이ymeriza- bility of MA by the silyl radical seems to be naturally higher than that of MMA. The sterically less bulky silane PhMeSiH2 afforded p이y(MA)s with somewhat higher molecular weights and with similar TGA residue yields compared with the ste
rically bulkier silane Ph2SiH2. The secondary silyl moieties, once attached to the poly(MA) as an end group, could be left untouched before the pyrolysis occurring at high tempe
rature. Other types of cross-linking process were suggested, based on the trend of the TGA residue yields. The secondary silanes appeared to competitively and concurrently function as both chain initiation and transfer agents in 사le photopoly
merization of MA.
Acknowledgment. This research was supported by the Korea Science and Engineering Foundation (1995).
Photopolymerization of MA with Secondary Silanes Bull. Korean Chem. Soc. 1996, Vol. 17, No. 6 535
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